From time to time, a user equipment (UE), such as a mobile phone or other remote terminal in a cellular radio communication system, searches for and selects cells and public land mobile networks (PLMNs). Cell and PLMN selection has a number of objectives, which include connecting a UE to the cell(s) and PLMN(s) that will provide the highest quality of service (QoS), enable the UE to consume the least power, generate the least interference, etc.
Wireless communication systems include time-division multiple access (TDMA) systems, such as cellular radio telephone systems that comply with the GSM telecommunication standard and its enhancements, such as general packet radio service (GPRS) and enhanced data rates for GSM evolution (EDGE); code-division multiple access (CDMA) systems, such as cellular radio telephone systems that comply with the IS-95, cdma2000, and wideband CDMA (WCDMA) telecommunication standards; orthogonal frequency division multiple access (OFDMA) systems, such as cellular radio telephone systems that comply with the Long Term Evolution (LTE) standard; and “blended” systems. GSM, WCDMA, LTE, etc. are different radio access technologies (RATs), and the Third Generation Partnership Project (3GPP) promulgates specifications for wireless communication systems that use such technologies.
When a UE performs a PLMN Scan, the UE typically carries out three steps: a received signal strength indication (RSSI) scan, cell search on chosen carrier frequencies, and a read of broadcast PLMN information where the cell search was successful. Cell selection is usually based on measured received signal strength (e.g., signal to interference ratio (SIR) or signal to noise ratio (SNR)) of candidate cells. Fast and efficient cell searches and received signal measurements are important for a UE to get and stay connected to a suitable cell, which can be called a “serving cell”, and to be handed over from one serving cell to another. On a regular basis, a UE measures its received signal strength and signal quality of each cell it detects, including the serving cell, to determine whether a change to a new serving cell is needed or not. In general, the new serving cell can use the same carrier frequency as the old serving cell or a different frequency.
For 3GPP-compliant wireless communication systems, the PLMN scan process is specified in, for example, Section 4.4 of 3GPP Technical Specification (TS) 23.122, Non-Access-Stratum (NAS) Functions Related to Mobile Station (MS) in Idle Mode (Release 6), V6.5.0 (June 2005); Section 5.1 of 3GPP TS 25.304 V6.10.0, UE Procedures in Idle Mode and Procedures for Cell Reselection in Connected Mode (Release 6) (March 2008); and Section 6.6 of 3GPP TS 45.008 V6.20.0, Radio Subsystem Link Control (Release 6) (February 2008). PLMN scan is also described in U.S. Patent Application Publications No. US 2002/0119774 A1 by R. Johannesson et al. for “Method for PLMN Selection” and No. US 2008/0153486 A1 by J. Ramkull et al. for “Efficient PLMN Search Order”. US 2002/0119774 describes how a UE can receive a list of data associated with networks neighboring the PLMN currently serving the UE from a base station (BS) of the PLMN currently serving the UE, and a new PLMN to serve the UE can be selected based upon the list US 2008/0153486 describes how a UE can shorten the time needed to find a cell, such as a suitable or acceptable cell, by using intelligent search orders.
For 3GPP-compliant wireless communication systems, procedures for selecting and re-selecting cells are defined in, for example, Sections 5.2 and 5.4 of 3GPP TS 25.304. Cell selection and re-selection are also described in U.S. Patent Application Publications No. US 2008/0031368 A1 by Lindoff et al. for “Efficient Cell Selection”; No. US 2009/0059871 A1 by Nader et al. for “Time-to-Trigger Handling Methods and Apparatus”; and No. US 2007/0121552 by B. Lindoff for “Cell Selection in High-Speed Downlink Packet Access Communication Systems”.
U.S. Pat. No. 7,013,140 to Östberg et al. describes cell searches that are based on a history list and cell planning knowledge. The history list is used when a PLMN scan is triggered, and frequencies in the history list are objects for cell search. For a WCDMA system, a frequency usually covers a large geographic area. The knowledge of cell planning involves the 5-MHz width of a WCDMA carrier and a rule that a network operator typically tries to fill up a frequency band completely. Carriers (cells) are then assumed to be located with 5-MHz relative distances from the first carrier placed on the lowest frequency of the band.
In many wireless cellular communication systems, each cell broadcasts information about its geographically adjacent cells, or neighbor cells. The information is typically broadcast as a Neighbor Cell List, which includes information needed by a UE to carry out the cell selection procedure on the neighbor cells. UEs typically maintain local copies of the latest-received Neighbor Cell List, and make received-signal measurements on the cells identified in the List. In some cases, a UE reports the results of its measurements to its serving cell, enabling the network to direct the UE to reselect to a neighbor cell, and in other cases, the UE on its own initiative reselects to a neighbor cell based on its measurements.
For 3GPP-compliant wireless communication systems, procedures for use of Neighbor Cell Lists are defined in, for example, Section 3.4.1.2 of 3GPP TS 44.018 V6.23.0, Radio Resource Control (RRC) Protocol (Release 6) (December 2007); and Sections 8.4.1.6.7 and 8.4.1.9.5 of 3GPP TS 25.331 V6.19.0, Radio Resource Control (RRC) Protocol Specification (Release 6) (September 2008).
For 3GPP-compliant wireless communication systems, procedures for network registration are defined in, for example, Section 4.4.1 of 3GPP TS 24.008 V6.19.0, Core Network Protocols, Stage 3 (Release 6) (June 2008); and Sections 4.1 and 5.2.2.1 of the above-cited 3GPP TS 25.304.
A Neighbor Cell List is a tool that a network operator can use to control a UE's movement through the network. For example, an operator can use a Neighbor Cell List to broadcast information about which and how many carrier frequencies the UE shall measure on.
Broadcast Neighbor Cell Lists are typically very operator-specific, which is to say that each network operator simply describes its own cell/frequency deployment in its Neighbor Cell Lists and disregards any user preferences. The typical disregard of user preferences causes problems, particularly in scenarios in which a UE has “roamed” from its “home” network run by its “home operator” into another network run by a “roamed operator”. To permit roaming, the roamed operator of the roamed-into network typically has a so-called roaming agreement with the home operator that allocates charges for services, etc.
For example, suppose the UE is in an area in which the home operator's network and the roamed operator's network overlap. The UE can be deemed to have roamed into the roamed operator's network when the UE merely temporarily loses coverage by its home operator's network. “Temporarily” can mean anything from a few seconds to a much longer period, e.g., days while a UE travels abroad. Once in the roamed operator's network, the UE sees only Neighbor Cell Lists that address the roamed operator's cells, and so the UE measures only these cells. It should be appreciated that this can happen despite a very short loss of coverage by the home network and despite the fact that the UE could find its home network if only the UE would search for the correct cells. The UE will not find the home operator's network again unless it is configured to scan for its home network every now and then.
Even if a UE is configured to scan for its home network, for example upon lapse of a suitable timer, the roamed operator may have entered the UE's home PLMN as an equivalent PLMN (EqPLMN). This effectively disables the UE's home PLMN scan timer, and the UE will have to lose coverage of the roamed operator's network before the UE will select a different PLMN, potentially its home PLMN.
Another problem in the example is that both the user and the home operator suffer extra monetary charges imposed by the roamed operator according to the type(s) of service(s) used. That can be particularly irritating for the user and the home operator when the UE is in an area in which the home operator's network and the roamed operator's network overlap, and the UE inappropriately gets and stays connected to the roamed operator's network.
Another problem in the example is that the roamed operator's Neighbor Cell Lists can point at cells and/or carrier frequencies and/or RATs that the UE is not permitted to use. For example, such cells/frequencies/RATs may be used by only the roamed operator's own subscribers. A cell search is a time- and energy-consuming procedure that can take up to 400 ms. As a result, the UE can waste time and electrical energy making vain received signal measurements.